Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives an inbound RF signal via the antenna and amplifies it. The one or more intermediate frequency stages mix the amplified RF signal with one or more local oscillations to convert the amplified RF signal into a baseband signal or an intermediate frequency (IF) signal. The filtering stage filters the baseband signal or the IF signal to attenuate unwanted out of band signals to produce a filtered signal. The data recovery stage recovers raw data from the filtered signal in accordance with the particular wireless communication standard.
The filtering stage of the receiver may either be a low pass filter or a bandpass filter. In either implementation, the filtering stage functions to pass signals of interest and to attenuate unwanted signals. Typically, the unwanted signals are adjacent channels and/or noise. To ensure that the unwanted signals are sufficiently attenuated, the filtering stage should have a sharp roll-off at its corner frequency or frequencies, the corner frequency or frequencies should be at the desired frequencies, and, for bandpass filters, the center frequency should be at the desired center frequency.
While these are the desired characteristics of the filtering stage, they are very difficult to obtain within radio frequency integrated circuits without some type of process variation compensation. As is known, CMOS technology process variations yield components that have a tolerance of +/−20%. Thus, for a filter that includes a resistor/capacitor (RC) stage, the corner frequency may be off by as much as 20%. If the filter includes multiple RC stages, which is the case to obtain a sharp roll-off, the corner frequency errors increase. For example, for a five RC stage filter, the error may be as much as 100%, yielding an entirely different filter than the one desired.
To compensate for integrated circuit process variations, the RC stages include tunable capacitors (e.g., switchable capacitor banks). In addition, the integrated circuit includes an on-chip calibration circuit that includes its own RC stage to tune the RC stage(s) (i.e., enable the switchable capacitor bank to approximate the desired capacitance). To determine a process error, the calibration circuit provides a pulse signal to its RC stage and measures the RC time constant. The measured RC time constant is compared to a desired RC time constant (which is readily calculable since the resistor and capacitor of the calibration circuit are designed to have a specific value) to determine the process error. Based on the process error, the calibration circuit tunes the RC stages of the filters (e.g., provides a signal which configures the switchable capacitor bank to yield approximately the desired capacitance).
While this approach mitigates the adverse effects of process variation on integrated circuit filters, for high performance circuits, such as an integrated Bluetooth radio, the reduced adverse processing variation effects are still too great. In addition, the unilateral calibration test assumes uniform process variations across the integrated circuit, which may or may not be the case, and adjusts all the components (e.g., capacitors) by the same amount. Thus, in a multiple RC stage filter, it is very difficult to accurately obtain the desired filtering response on a chip-to-chip basis.
Therefore, a need exists for a technique to directly tune a component embedded within an integrated circuit, especially components of a filtering stage in a radio frequency integrated circuit.